This invention relates to a transformer bobbin, in particular for supporting concentric transformer windings. In concentrically wound transformers, a primary winding is wound onto a bobbin, a layer of insulation is applied around the primary winding and then a secondary winding is wound around the primary winding. Such concentrically wound transformers have, in comparison with side by side wound transformers, certain advantages. In particular the leakage inductance can be much lower making the transformer easier to match, and to have a higher frequency response, a wider bandwidth and improved crosstalk characteristics.
Side by side transformers are often encapsulated to achieve the necessary levels of safety, whereas safety isolation is usually provided on concentrically wound transformers by other means. Encapsulation typically impairs the performance of the magnetic core such that concentrically wound transformers may often use less magnetic material than their (encapsulated) side by side equivalents.
In transformer manufacture, in particular in safety isolating signal transformers for use in telecommunications, the isolation of one winding from another is critical and has to be maintained under all conditions. These requirements are often difficult to achieve without adding to the complexity of the manufacturing process, and in particular it can be difficult to provide the necessary isolation for the entry and exit wires to/from the primary winding of a concentrically wound transformer, relative to the secondary winding which will be wound on top.
According to the invention, there is provided a bobbin for supporting concentric transformer windings, the bobbin having an area for receiving a primary winding, shoulders at each end of the primary winding area to define the space for the primary winding, an area for receiving a secondary winding surrounding the primary winding, and flanges at each end of the secondary winding area, wherein one of the shoulders bounding the primary winding area has a slot for the primary winding entry and exit wires, the slot extending across the shoulder from the primary winding area to the adjacent end of the bobbin, the slot communicating with an undercut region of the shoulder, below an upper surface of the shoulder, one lateral edge of the slot and the corresponding lateral edge of the undercut region diverging from one another in the direction of said adjacent bobbin end.
By feeding the primary winding exit and entry wires through an undercut region of the shoulder, the necessary distance through solid insulation between each part of the primary winding and any part of the secondary winding is achieved by the thickness of the shoulder, where it extends across the top of the undercut region. The diverging lateral edges of the slot and of the undercut region ensure that the primary winding exit wire not only can be wound by a conventional winding machine, but also can be reliably placed in a position where it will be separated from any part of the secondary winding by the necessary insulation, be it distance through solid insulation, creepage, clearance or thin sheet insulation.
It is normal for transformer bobbins with laminated cores to have a generally rectangular cross section, and in such a case the end of the slot adjacent to the primary winding area preferably lies on a corner of the bobbin rectangular cross section. Because the windings will always be pulled into closest contact with the bobbin and with the underlying windings at this point, this allows the exit wire to be taken off from the primary winding substantially level with the upper surface of the shoulder. Thus it is ensured that all the primary winding space is used and, for a given size of primary winding, allows the bobbin and thus the transformer to be of small dimensions.
The end of the slot remote from the primary winding preferably lies midway between the position which will be taken up by entry and exit terminals for the primary winding. The lateral edge of the undercut region preferably lies substantially parallel to the bobbin axis, and the lateral edge of the slot lies at substantially 45xc2x0 to the bobbin axis. The end of the slot adjacent to the primary winding area can be formed with a surface to engage and retain the wire as the wire passes over it, to hold the wire against axial tension. The undercut preferably extends laterally on both sides of the slot and both lateral edges of the undercut region lie parallel to the bobbin axis. The edge of the slot on the exit wire side of the slot is chamfered.
Mountings for both primary and secondary winding terminals can be located radially outside the winding spaces so that there is sufficient creepage distance between the entry and exit wires and the magnetic core, with primary winding terminals being at one end of the bobbin and secondary winding terminals at the other end.
If the primary winding area is located axially centrally relative to the secondary winding area, an advantageous, space-saving arrangement is achieved.
The invention also extends to a concentrically wound transformer formed on a bobbin as set forth above.
According to another important and separate aspect of the invention, there is provided a housing for an electronic component comprising a box-like enclosure open at one side, snap-fit securing means for enabling a component to be fitted into the housing and retained in the housing, and openings through the housing walls at edges and/or corners of the housing. This aspect of the invention extends to the combination of an electronic component and a housing therefore, the housing comprising a box-like enclosure open at one side, snap-fit securing means in the housing, snap-fit features on the component, the snap-fit securing means and the snap-fit features enabling the component to be snap-fitted into the housing and retained in the housing, and openings through the housing walls at edges and/or corners of the housing.
This type of construction has many benefits when compared with the conventional xe2x80x98pottingxe2x80x99, xe2x80x98encapsulationxe2x80x99 or xe2x80x98varnish impregnationxe2x80x99 processes. Assembly of the component into the housing now requires only one mechanical action and provides all the required electrical insulation and isolation for the component. Because there is no encapsulating or varnish compound consolidating the component, disassembly and recycling of the materials from which the component is made is easily possible. When the component is a transformer as described here, the plastics housing can be broken to remove the transformer. The laminations can be extracted, as they are only held in position by the housing, and the copper wire can be unwound from the bobbin. The plastics bobbin and housing can be recycled using conventional plastics recycling techniques; the copper wire can be recycled after remelting and the laminations can be reused without any further treatment (or can be recycled after remelting). The material of the laminations is expensive, and this recycling can be worthwhile if only to recover and reuse the laminations. None of this would be feasible with an encapsulated, varnished or over-moulded component which would have to be disposed of in landfill.
Thus, according to a further aspect of the invention, there is provided a housing for an electronic component, the housing having a set of walls for surrounding the component and lugs past which the component can snap to retain the component in the housing.
The housing will preferably have five walls (i.e. an open bottom through which the component can be introduced), and openings at or near the apexes of the housing.
The housing will be particularly suitable for components which are to be mounted on a printed circuit board.
The invention will now be further described, by way of example, with reference to the accompanying drawings in which: